Bright metallic finishes are used in applications requiring a high degree of reflection over wide wavelengths. These finishes are typically used in decorations and ornamentations for aesthetic value. Packages with these brilliant metallic finishes outperformed those without metallic enhancement in 80% of the test cases in marketing study (Brand Packaging, Jan. 1, 2004).
It is often desirable to spot metallize (also known as spot application) a portion of a package. The metallic finishes are typically transferred from a two-dimensional sheet or web of metallized films, metallized papers or metallized foil onto the packages. However, creating spot metallization from the two-dimensional sheet creates wastes and additional steps. In one method, the metallized film is hot-stamped on designated areas of the package. In another method, the entire surface of the package is first covered with the two dimensional metallized film, and then portions of the metallic area is covered with a layer or layers of high opacity white ink to coat over the metallized portions. Both of these methods increase the overall cost, time and waste.
Alternatively, metallic particles are incorporated in a solvent-based binder system. However, solvent-based metallic coatings are not preferred in high production and large scale operations because they must be physically dried or heat cured, often incurring large amounts of energy, time and cost. Mills (U.S. Pat. No. 4,233,195) teaches a metallic ink composition; however, this composition is a pasty solvent-based ink that must be applied through an offset ink station at 300° F. heated roller to create a metallic paper. Kruger et. al., (US 2008/0131383), describe an in-situ solvent-based resin binder system that includes physical vapor deposition aluminum flake and a leafing additive to form artificial nails; however, the resin binder system must be physically dried to form the metallic effects. Volt et. al., (US 2010/0064938), describe a water polymer and/or organic binder solvent system containing silver-dollar leafing aluminum and an organofunctional siliane to create a high brilliant metallic finish. Again, this is a solvent and water based system, which must undergo a drying process.
Maintaining high gloss for metallic surfaces can be challenging in a water based system. Low et al., (US 2010/0151139) describe an aqueous polyurethane based metallic coating that can be physically and/or thermally curable with PVD aluminum flakes. However, the gloss level is less than 104 gloss units (GU), which is less than high brilliant finish (typically 190 GU or higher as measured with a 60 degrees gloss meter).
While a system without physical drying or thermal curing is desirable, a balance of fast cure speed and shelf-stability is a challenge, especially for aluminum and copper metal flakes. Ikeya et al., (U.S. Pat. No. 7,837,777) describe the use of nitrocellulose to provide shelf stability and to stop premature gellation for a surface treated metal flakes in a UV-curable metallic inks, but nitrocellulose fails to contribute to cure speed enhancement.
To date, only a limited number of combinations of components are known to create a high gloss metallic effect for UV curable composition. Krohn (U.S. Pat. No. 6,805,917) teaches a UV-curable system that utilizes Novolac epoxy acrylate with isobonyl acrylate and isobonyl methacrylate. A flow additive, ethyl acrylate/2-ethylhexyl acrylate copolymer (Modaflow), is further added to metallic pigments. While the cure speed of the composition is adequate due to high initiator loading, Krohn is silent as to the gloss measurement of the finish.
There remains a need in the art for energy-efficient and environmentally-friendly metallic compositions, which are UV-curable in high speeds, shelf-stable, and can be cured to high gloss levels. The current invention fulfils this need.